The invention relates generally to the art of injection molding and more particularly to methods and apparatus for sensorless observation of melt pressure in an electric injection molding machine.
Injection and other types of molding machines are complex systems, typically operated in multiple steps or phases, in order to provide a molded part or parts in a molding cycle. Once a finished part is ejected or removed from the machine, the molding cycle is repeated to produce further parts. A typical injection molding machine operational cycle includes clamp, inject, pack and hold, recover, and eject steps, each of which involves moving machine components and motion control thereof. The clamp phase joins the individual sides or portions of a mold together for receipt therein of pressurized plastic molding material, in the form of a melt. In the inject phase, a reciprocating feed screw or ram within a cylindrical barrel pushes or injects the plasticized melt through an orifice at the barrel end or nozzle, which in turn provides the melt to the interior cavity of the mold. Further material is then provided to the mold and pressure is maintained during a pack and hold step, and the eject step separates the molded part from the separated mold halves. The screw is retracted in the barrel during a recovery step while the screw is rotated to advance new plastic material through screw flights into the barrel space forward of the screw, whereupon the cycle may be repeated.
In the United States, injection molding machines have traditionally been hydraulic machines. Within the last several years, there has been a general shift to electrically powered machines either in hybrid arrangements (where some machine functions are performed by electric motors while others hydraulically) or in an all-electric machine. The machine cycle is the same whether the injection molding machine is electrical or hydraulic. However, there are major differences in the hardware required to perform the sequences in the molding cycle. The hardware differences require control changes to the machine so that the user of an electric injection molding machine can perform the same types of control traditionally performed when molding with hydraulic machines.
Controls for injection molding machines have evolved from early manual controls wherein plastic was injected into a mold when a crank wheel was turned, to programmable logic controllers operating the machine actuators in closed loop fashion using sensor inputs to implement a control law, typically proportional, integral, derivative (PID) control. More recently, molding machine controllers have provided more advanced functionality, such as combining auto-tuned PID with a predictive open-loop term and an adaptive learned disturbance correction term, as set forth in my U.S. Pat. No. 5,997,778, the disclosure of which is hereby incorporated by reference as if fully set forth herein. The present invention provides various improvements over the conventional molding machines and control systems therefor illustrated and described in U.S. Pat. No. 5,997,778.
Of particular importance is the motion of the in-line reciprocating screw or ram during injection and also the control of the ram during hold and pack. This is necessary to achieve acceptable molded parts, for example, by insuring complete filling of the mold, reducing cosmetic and structural problems in the molded parts, ejecting molded parts without damage and minimizing cycle time to achieve acceptable machine throughput.
Control of the ram motion during inject is typically accomplished in one of two ways. In a first method, the velocity of the ram along its travel is set at desired velocities at desired travel distances by the operator. The velocity of the ram is thus profiled and typically the control monitor will show the programmed velocity as a trace and the actual trace achieved during injection. When velocity profiling is used, the machine transitions from a motion control to a pressure control during hold. That is, the shrinkage of the melt while the plastic starts to cure is made up of additional plastic pushed into the mold as a function of the packing pressure desired to be exerted on the plastic in the mold. Still further, when velocity profiling is used, the machine control will monitor and can display the pressure on the ram, sometimes in an overlaid display, to allow the operation to better set the velocity profile. In any event, the pressure is typically monitored and values displayed when velocity profiling is used.
The second method to control the ram is by pressure and not velocity. Pressure profiling is similar to velocity profiling in that the operator sets pressure at specified ram positions to achieve desired profiling. While velocity profiling is widely used, there are applications where pressure profiling is desired. For example, gas injection mold technology may require ram pressure control to achieve a desired gas pressure in the mold.
In a conventional hydraulic machine, an inexpensive pressure sensor(s) is located in the hydraulic circuit. The hydraulic pressure is directly sensed and a good portion of the entire control system is governed by the pressure in the hydraulic circuit. Because of the mechanical arrangement used to mount the hydraulic actuator, i.e., piston, a direct correlation between the pressure used to push the ram and the pressure in the mold is obtained. Unless an intricate mold is used, the hydraulic pressure on the ram can be easily correlated to the pressure of the melt in the mold. Thus, pressure profiling is provided at no cost in a hydraulic machine. The hydraulic circuit sensor needed to operate the machine provides the sensor information for machine control.
An entirely different problem is present with an electric machine. First, the rotation of an electric motor has to be converted to linear motion, typically accomplished by a ball screw. To permit translation and rotation of the ram (screw), a tie-bar structure, not dissimilar to that used for the platens of a mold is typically employed. The ball screw pushes a plate sliding in a guided manner on the tie-bars secured to the ram (screw) to achieve translation. This mechanical arrangement does not provide the inherent pressure reading available in a hydraulic machine to sense pressure. What is done then is to provide a melt pressure sensor in the barrel or mold to sense the pressure. However, melt pressure sensors do not operate in the pristine environment of the pressure sensors in the hydraulic circuit where the sensor is exposed to clean fluid at low temperatures. Basically, the melt pressure sensor is an expensive instrument with a short finite life. Further, and somewhat surprising, the melt pressure sensor may not necessarily give a true accurate or absolute reading of the actual pressure in the mold and may not have sensitivity or response time to truly pick up variations in the melt. However, in most cases, the melt sensor will give a consistent reading. Furthermore, if a melt sensor is used on the machine and not the mold, special nozzles or nozzle adapters are frequently required to mount the pressure transducer in the machine barrel, and, in all events, the transducer needs to be calibrated upon installation and periodically thereafter. Calibration and changeout of failed barrel mounted pressure sensors are off-line procedures causing machine down-time, and are typically beyond the expertise of mold operators. This is in addition to external signal conditioning and/or amplification. Thus, several shortcomings make direct transducer measurement of melt pressure less than desirable.
The prior art has recognized this and has used a force transducer (i.e., strain gage, load cell) to measure the axial force executed on the ram (typically, vis-a-vis the mounting plate). See, for example, U.S. Pat. 4,828,473, 4,961,696 and 5,955,117.
In the prior art, for example, strain gage sensors or load cells are mounted so as to measure compression on the ram or some other component in line between the linear (e.g., motorized) actuator and the plasticized melt. The compression of the ram, for instance, is generally related to the strain at the measurement point, and is also roughly proportional to the total force thereat. Because the majority of the force on the ram is due to the melt pressure resistance, such strain gages have heretofore been used to provide an estimate of the melt pressure. However, this technique does not provide exceptional accuracy, due to the inability to discriminate between melt pressure and other forces on the ram.
More particularly, the movement of the ball screw actuator encounters frictional forces which are not necessarily consistently repeatable in magnitude. In addition, there is a stretch or deformation of the mechanical components, such as the tie rods, which are also not necessarily consistently repeatable. These inconsistent variables are inherent to any mechanical system in the electric drive inject mechanism. This means that a force transducer in the electric drive machine cannot give consistent readings in contrast to, say, the consistency of the readings of melt or liquid pressure sensors.
Further, load cells and strain gages require sensors and high-gain amplifier components. Moreover, the relationship between force and the resulting deflection at the strain gage is typically non-linear, resulting in inaccuracies at certain regions of pressure operation. Thus, strain gage and load cell techniques suffer from high cost, lack consistency, and do not provide accuracy required for improved molding machine control, particularly during peak velocity or high pressure conditions.
Whether direct pressure sensor transducers or strain gages are employed in conventional molding machines, signal conditioning and amplification circuitry must be provided, in order to convey a pressure indicative signal to the machine control system. Such circuitry adds to the cost of operating the machine. Furthermore, such circuitry typically suffers from drift over time, and which may vary due to fluctuations in operating temperature. Moreover, since strain gage amplifiers are typically high gain devices, electrical noise (e.g., EMI or RFI) susceptibility may cause inaccurate or unusable pressure measurements, particularly in the presence of rotating electrical machinery (e.g., such as electric motors in an electric molding machine) or power supplies therefor (e.g., variable speed motor drives and the like). Although filtering techniques can be employed to reduce the noise susceptibility in such amplifiers, such filtering reduces the response time of the sensor signal circuitry, resulting in signal latency which may be unacceptable in attempting to attain precise control of the molding machine injection step.
It should also be noted that the electrical drives of the electric injection molding machine motors control the motor by torque or torque limiting controls. Specifically, it is known to control pack pressure by limiting the torque value of the motor correlated to maximum pack pressures set by the operator. Within the prior art, there are certain control schemes that transition the machine from velocity control to torque limit control for pack to effect a xe2x80x9cseamlessxe2x80x9d transition. That is, the motor can be set to reach a pre-set torque believed adequate to produce a desired pack pressure which is, generally speaking, steady state. Since the motors will ramp to the torque limit, transitions are provided. While the machine controller can be programmed to provide the torque limit command to the drive, there is no ability to provide a command to the drive which varies to account for the system variables to produce a desired pressure unless the pressure is directly sensed. As a general observation, it is noted that while the drive for the motor will provide an instantaneous read of the motor current draw, there is no attempt to analyze that signal to determine what portion is attributed to pressure. Simply reading a current or controlling the motor current, without more, will not suffice.
Thus, conventional melt pressure measurement and estimation techniques and systems do not provide optimal indication of the actual melt pressure so as to allow improved control of injection molding machines. Consequently, there remains a need for improved apparatus and methodologies by which improved control over, and monitoring of melt pressure can be achieved, without adversely impacting molding machine cost and operational maintenance.
The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. It is intended neither to identify key or critical elements of the invention, nor to delineate the scope of the invention. Its primary purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later. The present invention involves techniques and apparatus for sensorless control and monitoring of melt pressure in injection molding machines, by which improved control capabilities and reduced cost can be achieved, in comparison to conventional melt pressure sensing and estimation techniques. In this regard, the invention facilitates mitigation or avoidance of the above mentioned and other difficulties associated with direct or indirect melt pressure measurement and/or control in the prior art (e.g., such as pressure transducers, strain gages, load cells, and the like).
The invention relates to methods and apparatus for estimating or observing melt pressure in an injection molding machine, without the need for direct measurement thereof. In particular, the invention finds application in association with electric or hybrid electric molding machines, wherein hydraulics are not used to actuate the ram during an injection step. Employment of the various aspects of the invention further allows elimination of costly and often difficult to maintain sensor and pre-amplifier components (e.g., and/or strain gage devices) from injection molding machines. Additionally, the invention provides for melt pressure control and/or monitoring in the situation where a barrel or cavity mounted pressure transducer has failed, and does not suffer from the noise susceptibility issues present in conventional molding machines. Furthermore, the invention provides melt pressure estimates where frictional and other loss forces have been taken into account, such that the melt pressure value provided is a better estimate of the actual melt pressure than was previously possible. Thus, the invention provides significant advantages over conventionally employed pressure melt measurement techniques and devices.
In accordance with one aspect of the present invention, there is provided injection molding machine apparatus and controls therefor, having a model representative of the machine behavior and a melt pressure observer. The model may comprise one or more state equations, some or all of which may be differential equations. The observer component receives one or more input values from the injection molding machine or from devices associated therewith, and uses these to provide an estimate for the melt pressure using the model component. For instance, the model may comprise a melt pressure equation expressed as a function of motor acceleration and torque associated with an electric translational drive motor. A first input may represent motor velocity or position and a second input value may represent motor current or torque, wherein the observer provides a current melt pressure value according to one or both of the first and second input values using the model. The current melt pressure value is thus obtained without directly sensing the melt pressure, whereby an injection molding machine may be operated so as to control melt pressure, without the cost, maintenance, and other shortcomings associated with conventional melt pressure sensing techniques. The first and second input values may be obtained from dedicated sensors associated with the molding machine, and/or directly from a motor drive associated with the translational electric motor. Thus, the invention may be employed to facilitate sensorless pressure control or monitoring in injection molding machines.
The melt pressure value so obtained may be used in controlling the machine, for monitoring purposes, and/or combinations thereof. For example, during an injection step of a molding machine cycle, velocity profiling may be performed for an initial boost period to a first cutoff point while melt pressure is monitored to avoid exceeding a preset pressure limit value. Thereafter in a pack/hold step, pressure may be controlled according to a pressure vs. time profile, wherein the observed melt pressure value is provided as feedback to an injection machine control law, which in turn provides a control output to a drive motor associated with the injection ram. In this regard, a control system operatively coupled with a motor drive to control the longitudinal translation of the injection ram may comprise a control law providing a control signal to the motor drive according to a current machine state so as to effectuate a desired translation of the injection ram in a controlled fashion. The current machine state may comprise the current melt pressure value, wherein the control law provides the control signal to the motor drive according to the current melt pressure value. In this manner, pressure control can be achieved without pressure sensing. The present invention advantageously provides an indication of the melt pressure without the use of external sensors or strain gages, and the cost and noise sensitivity issues associated therewith. In addition, melt pressure can be monitored or used for control where one or more such sensors have failed.
The model may include one or more state equations, which are solved by the observer component to yield the current machine state. For example, a melt pressure equation may be provided, which is a function of motor acceleration and motor torque. The melt pressure observer solves the melt pressure equation using the first and second input values to provide the current melt pressure value. Thus, where the first input value represents the drive motor velocity, the acceleration can be derived using differentiation, which is then used to solve the melt pressure equation. The differentiation may be accomplished using various digital filtering techniques, such as a Hamming window or other numerical methods. Where the first input value is a position indication, the velocity and ultimately the acceleration can be derived therefrom for use in the melt pressure equation. Similarly, where the motor drive or other sensor provides a second input value indicative of motor torque, this can be directly used to solve the melt pressure equation. Alternatively, where a motor current indication is available (e.g., from a dedicated sensor or from the motor drive), motor torque can be derived therefrom, such as by using a lookup table or a torque equation in the model. The model and the observer, moreover, can account for non-linearities, noise, or other physical characteristics of the molding machine to provide a more accurate indication of the actual melt pressure than was possible using direct sensing techniques. For example, the melt pressure equation can take into account frictional forces associated with the translation of the injection ram. Thus, where a direct sensor cannot differentiate between such frictional forces and the actual melt pressure forces, the invention allows such forces to be subtracted or otherwise accounted for before rendering a melt pressure value.
Another aspect of the present invention involves methods for estimating a melt pressure associated with the longitudinal translation of an injection ram in an injection molding machine having an electric motor. The methods may be implemented, for example, in control systems associated with electrically driven injection molding machines, including but not limited to object oriented software implementations in PC based machine control systems, and/or combinations of hardware and software. The methods comprise obtaining a first input value indicative of a velocity or a position associated with an electric motor, and a second input value indicative of a current or a torque associated with the electric motor. One or more state equations are then solved according to the first and second input values in order to provide a current melt pressure value. The melt pressure value can be used for monitoring or for controlling the molding machine operation. Where pressure is being controlled in the machine, the method can further comprise providing a control signal to a motor drive associated with the electric motor according to a current machine state so as to achieve a desired translation of the injection ram in a controlled fashion, where the current machine state comprises the current melt pressure value. The invention further comprises computer-readable media having computer-executable instructions for controlling the longitudinal translation of an injection ram in an injection molding machine using an electric motor, by which sensorless indication of melt pressure can be facilitated.
It is a particularly important feature of this invention to provide in an all electric molding machine where the screw or ram is translated by an electric motor through a mechanical drive arrangement, a control system which does one or more of the following:
1) senses pressure indicative of the melt pressure without having a force transducer or pressure transducer sensing the force on the screw or the pressure on the melt;
2) provides pressure profiling without using the force transducer or pressure transducer in item 1; and/or,
3) provides hold and pack and transition to a set pressure relative to existing pressure without sensing existing pressure by a force transducer or pressure transducer.
Another aspect of this invention resides in its ability to provide a control system which can consistently sense melt pressure in an electric machine with a mechanical drive notwithstanding variations that occur in the mechanical drive during its operation even if the same operation is repeated.
Still another aspect of this invention resides in a control system which, without the presence of a melt transducer or a force transducer measuring a force indicative of a melt pressure, is able to provide a measurement indicative of the melt pressure which is consistently sensitive and responsive to changes in the melt pressure to control the machine.
To the accomplishment of the foregoing and related ends, the following description and annexed drawings set forth in detail certain illustrative aspects and implementations of the invention. These are indicative of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.